As this poster explains, it doesn't work how you seem to think:
Hereward1332 Thu 02-Apr-20 14:40:33
They didn't decline to buy them. They weren't on offer. They didn't join a joint procurement scheme, where all members of the scheme would source whatever ventilators they could, and allocate them amongst scheme members, presumably according to need.
This could have seen the UK get less than it wanted as the needs of other countries (Italy, Spain) are greater. Whether it is moral to try to buy more when other nations' suffering is greater is a different question, but the government is answerable to the UK public and believes this is the most likely path to obtaining the best best result for them.
Hope that helps.